Handover or handoff refers to a procedure supporting smooth maintaining of communication of a terminal moving between cells during call communication. Handover is classified into three types: soft handover; softer handover; and hard handover. Soft handover refers to handover performed while signals are simultaneously received from several base stations. Softer handover refers to handover performed between different sectors of the same base station, during which signals from the different sectors are simultaneously received. Hard handover refers to handover performed while selectively receiving only one base station signal at a moment.
For example, IEEE 802.16e standard defines four handovers: hard handover (HHO), optimized hard handover (OHHO), fast base station switching (FBSS), and macro diversity handover (MDHO). In this case, however, only the HHO and OHHO are included as essential functions in a system profile defined by WiMAX Forum for mobile WiMAX authentication.
The HHO is a scheme for releasing a connection with a base station on service (i.e., a serving base station) before the terminal establishes a connection to a new base station (i.e., a target base station) like the existing hard handover scheme, which can be understood as a handover scheme of ‘break before make’, ‘break before entry’ or ‘break before establishment’. The OHHO is a scheme optimizing the HHO, which enhances a service continuity and minimizes a call interruption or the like by reducing a handover switching time by optimizing the general HHO. The MDHO is a scheme for establishing a connection to a new base station before the terminal releases the connection with the current base station like the existing soft handover scheme, which can be understood as a handover scheme of ‘make before break’, ‘entry before break’ or ‘establishment before break’. The FBSS is a handover scheme having a medium performance and complexity of those between the HHO and the MDHO.
Meanwhile, IEEE 802.16m is a standard aiming at satisfying all the advanced requirements for IMT-Advanced system, the next-generation 4G mobile communication standard, while maintaining the compatibility with the IEEE 802.16e system. When the compatibility between the legacy system and the advanced system is to be maintained, mobility of the terminal between both systems is a major technical issue. For example, in an environment in which an IEEE 802.16e base station (i.e., a legacy base station (BS)) and an IEEE 802.16m base station (i.e., an advanced BS) coexist, handover between the IEEE 802.16e BS and the IEEE 802.16m BS must be smoothly performed. In particular, handover must be smoothly performed when the IEEE 802.16m BS does not support the IEEE 802.16e system.